Circuit breaker



Jan. 15, 1963 G. R. THOMAS ETAL 3,073,925

CIRCUIT BREAKER 2 Sheets-Sheet 1 Filed June 10, 1959 Jan. 15, 1963 G. R. THOMAS ETAL CIRCUIT BREAKER Filed June 10, 1959 Fig.7

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2 Sheets-Sheet 2 3,673,925 (IERCUIT BREAEQER Grenen R. Thomas, Beaver, and Robert Irl. Flick, Brighton Township, Beaver County, Pa., assignors to iVest-Lingn house Eiectrie Corporation, East Pittsburgh, Pa., a corporation or Pennsylvania Filed .lune itl, 1959, Ser. No. @9,263 9 Claims. (rCl. 20d-88) This invention relates to circuit breakers and more particularly to circuit breakers embodying electromagnetic tripping means.

An object of the invention is to provide a circuit breaker embodying a time-delay and instantaneous tripping mechanism with improved means for adjusting the instantaneous tripping characteristic of the circuit breaker.

Another object of the invention is to provide a circuit breaker embodying an electromagnetic tripping means having a lixed magnet yoke and a movable armature with improved means for adjusting the initial position of the armature relative to the liked yoke to vary the minimum overload current required to instantaneously trip the breaker.

One improvement obtained by this invention is increased accuracy and stability of the adjustment of the magnetic trip. One factor in obtaining this improvement is that the rotatable cam member which effects the adjustment is held by a rigid bracket against movement in the direction which would aiiect the calibration. Another improvement is that the breaker cannot be inadvertently tripped during the adjustment because inward movement of the adjusting member or knob which is engaged manually or by a tool does not cause tripping of the breaker, or any change in the magnetic trip setting, if the knob is not rotated.

These improvements are obtained in a multipole trip device which is covered and removable as a unit from the circuit breaker, and the adjusting members are at the top of the trip unit. The cover of the circuit breaker has openings over the adjusting members, so that the adjustment may be made without removing either the circuit breaker cover or the trip unit cover. At the same time, the thickness of the trip unit (in the direction of the length of the circuit breaker) is kept to a minhnum, so as to be usable in breakers only about two-thirds as long as those formerly used, of the same rating.

The invention both as to structure and operation, together with additional objects and advantages thereof, will be best understood from the following detailed description thereof when read in conjunction with the accompanying drawings.

In said drawings:

FIG. l is a vertical sectional view of a circuit breaker embodying the principles of the invention;

FIG. 2 is an elevational view of the trip device with the cover of the trip device broken away to better show the device;

FIG. 3 is a sectional view on an enlarged scale of the trip device;

FIG. 4 is an enlarged view of the adjusting cam and knob assembly;

FIG. 5 is an elevational sectional view showing the tripping magnet and adjusting means therefor for one of the poles of the breaker;

FIG. 6 is a sectional view taken on line VI-VI of FIG. 5 and looking in the direction of the arrows, the trip bar being omitted for the purpose of clarity;

FIG. 7 is a sectional view taken generally on line VII-VII on FIG. 5, but `showing certain parts also shown in FIG. 6, and

FIG. 8 is a fragmentary plan view showing the adjusting scale for one of the poles of the breaker.

Referring lto FIG. 1 of the drawings, the circuit breaker comprises generally a base 11 and a cover i3 both of molded insulating material. The breaker is of the two or three-pole type, each pole being provided with a line terminal l5 and a load terminal 17 at opposite ends of the base 1l.

The circuit breaker includes a stationary contact 21, a movable contact 23 and an arc extinguisher 25 for each pole. A common operating mechanism indicated generally at 27 is provided for simultaneously actuating the three movable contacts to open and closed positions, and a trip device indicated generally at 2@ serves to elect automatic opening of the breaker contacts in response to predetermined overload conditions in the circuit through any pole ofthe breaker.

The line terminal l5 is at the outer end of a conducting strip 31 which extends into the housing and rigidly supports the stationary Contact 2l. The movable contact 23 for each pole is rigidly mounted on a contact arm 33 supported on a switch arm 35 secured to a tie bar 37 which extends across all of the poles or" the breaker and supports the switch arms for the several poles of the breaker for unitary movement to open and closed positions. lThe contact arrn 33 is connected by means of a flexible conductor 39 to an intermediate or trip unit terminal 4l. (FIG. 3) secured to the base li by a screw 42, which also secures one end or leg of a heater element 43 to the base. A bimetal element i5 forming a part of the trip device 29 is mounted on the other leg of the heater element 43 which is electrically integral with the load terminal l'.

The operating mechanism 27 (FIG. l) is disposed in the center compartment of the housing and is supported by a pair of spaced frame members 53 (only one being shown) secured to the base ll. The operating mechanism comprises a pivoted forked operating lever S5, a toggle comprising toggle links 57 and 59, an overcenter spring means 6l and a pivoted releasable cradle 63 controlled by the trip device 23. An arcuate insulating shield 67 for substantially closing an opening 69 in the cover 13 is mounted on the end of the operating lever 55 and has an integral handle portion 7l extending out through the opening 69 to permit manual operation of the breaker.

The toggle links 57 and 59 are pivotally connected together by a knee pivot pin 73. .The toggle link S7 is pivotally connected to the cradle 63 by a pin 7S and the toggle link 59 is pivotally connected to the switch arm 35 for the center pole by a pin 77. The overcenter spring el is connected under tension between the knee pin 73 of the toggle 57-59 and the outer end of the operating lever 55.

The circuit breaker is manually operated to the open position by movement or the handle 71 (FIG. l) in a clockwise direction to the off position which movement actuates the overcenter spring 6l to cause collapse of the toggle 57-59 and opening movement of the switch arms 3S for all of the poles of the breaker in a wellknown manner.

The circuit breaker is manually closed by reverse movement of the handle 71 from the oil to the on position which causes the spring 61 to move overcenter and straighten the toggle, thereby moving the switch arms 35 for all of the poles to the closed position.

In addition to the bimetal element 45, the trip device includes a series overload tripping electromagnet 79 (FlGS. 2, 3 and 5) for each pole and a trip bar 8l common to all of the poles and biased to latching position by a spring (not shown). The trip bar 81 is made of insulating material and is pivotally supported by means of pins 33 (only one being shown in FlGS. 2 and 3) 93 in a well-known manner.

then closed in the previously described manner by movequired to instantaneously trip the breaker.

3 supported in a bracket 35 secured to the base 11 by the screws 42.

The bracket 85 also supports an insulating base 87 for the trip device. A cover 89 of insulating material is provided to enclose the trip device and is sealed to the trip unit base '17 of the trip device. The trip unit cover 39 is provided with three cut-out portions 91B to permit it to be assembled downwardly over the three line terminal strips 17.

A latch 91 on the trip bar d1 normally engages a latch mechanism 93 which, in turn, engages and releasably restrains the cradle 63 to releasably hold the operating mechanism in closed position.

The tripping electromagnet 79 comprises a fixed laminated, U-shaped magnet yoke 95, the laminations being secured together by rivets 9d (FIGS. 2 and 5). The magnet yoke 95 is iixedly connected to a bracket 97 (FlG. 3) by means of rivets 96 which rivets support the bracket 97 and magnet yoke 96 on the base 87 for the trip device. A movable armature 99 is pivotally supported by means of a knife-edge pivot 101 in a V-shaped notch 102 in the lxed yoke 95 and has pivotally con nected thereto a rod 1113 which, at its other end, has a head 1115 for engaging and operating the trip bar 81 upon energization of the magnet. Springs 107 are connected under tension between inwardly extending ears 1118 (FIG. 6) on the outer laminations of the armature 99 and inwardly extending ears 11@ on the outer laminations of the magnet yoke. The tripping magnet is energized by the terminal conductor 17 which extends between the legs of the U-shaped magnet yoke 95.

When a low overload current occurs, the bimetal element t5 is heated by the heater element and when heated a predetermined amount bends toward the right (FIG. 3) to engage the end of a Calibrating screw 109 in the trip bar 31 and actuates the latter to effect release of the cradle 63 .and opening of the breaker contacts after a time delay in a manner well known in the art.

The circuit breaker is tripped open instantaneously by operation of the tripping magnet 79. When the magnet is energized by overload currents of, for instance, 1000% or more of normal rated current, or by a shortcircuit current, the ,armature 99 is attracted causing the head 1115 of the rod 1113 to engage and actuate the trip bar 31, thereby effecting instantaneous release of the cradle 63. This effects automatic opening of the contacts for all of the poles of the breaker.

1t is necessary to reset .and relatch the breaker mechanisrn following an automatic opening operation before the contacts can be closed. Resetting and relatching is effected by moving the handle clockwise (FIG. 1) as far as it will go. During this movement, a projection 111 on the operating lever 55 engages ya shoulder on the cradle 63 and moves the latter in a clockwise direction. Near the end of this movement, the free or latching end of the cradle 63 is reengaged with the latch mechanism The breaker contacts are ment of the handle to the on position.

Means is provided to selectively determine within a predetermined range the minimum overload current re- This is `accomplished by adjusting the position of the armature relative to the magnet yoke' to thereby vary the magnetic air gap. The outer laminations of the magnet yoke which are thicker than the inner laminations have inwardly and upwardly formed portions 113 (FIGS. 2 and 5) which at their upper ends are formed over as at 115 and extend adjacent the free end of the armature 99. The projections 115 form auxiliary pole pieces which, when the magnet is energized, exert a back pull on the armature 99. The purpose of the auxiliary pole pieces 115 is to provide limited paths for `at least a portion of the magnetic flux to thereby provide a differential pull on the armature which may be'varied according to the adjustment of the armature relative to the yoke member 95 and the auxiliary pole pieces 115.

The armature 99 is adjusted by means of a lever 119 (FIG. 5) which is controlled by a manually set cam 121. The lever 119 is pivotally mounted by a pivot pin 123 on an L-shaped bracket secured to the magnet yoke 95 and having a portion 127 extending upwardly therefrom. The lever 119 at one end is provided with a nose portion 129 which engages `a stepped cam surface 13st? of the cam 121. The other end of the lever 119 is provided with an ear 133 which extends above and adjacent to the armature 99. A Calibrating screw 135 in the ear 133 bears against the armature 99. By turning the Calibrating screw 13S, the calibration of the magnetic trip may be set at the factory, and the setting of the screw 135 may then be sealed4 This setting would normally be made with the externally manually set cam 121 in its high (HI) position and fixes the top limit of magnetic trip operation so that it cannot be changed in use to be so high that effective instantaneous protection of the circuit against heavy overloads is not obtained.

The bracket 97 (FlGS. 3, 6 and 7) which supports the magnet yoke 95 has an upwardly extending portion 137 which is formed over at right angles to provide a support 139 for the bottom of the cam 121. The upwardly extending portion 127 of the bracket 125 is formed over at right angles as at 141, and an adjusting knob 142 is rotatably mounted through a hole in the portion 141. As best shown in FlG. 4, the knob 142 has a flange 144 which engages the under side of the supporting portion 1-11 to prevent the knob from moving outwardly. The knob 142 has a non-circular hole 11% such as the D-shape shown for receiving the upper end of the shaft 143, which may be flattened on'one side 146 so as to be keyed to the knob and turn with it.

rEhe cam 121 is integral with or otherwise rigidly secured to the lower end of thhe shaft. The cam 121 rests on the rigid portion 139 of the bracket 97 and the pivot portion Ill' in line with the end of the'shaft 143 extends through an opening inthe support or portion 139 to act as a pivot for the cam 121.

VA compression spring 149 positioned in the recess or hole 148 in the knob 142, biases the knob upwardly and holds the flange 141i against the underside of the bracket portion 1411. At the same time, the spring 149 biases the upper end of the shaft 143 downwardly and holds the bottom of the cam 121 against the rigid support 139. Thus, the cam 121, rod 143 and knob 142 assembly is held against longitudinal movement between the support 139 and the portion 141 of the bracket 125 and thus is not subject to longitudinal displacement in response to shocks. This construction is low cost in assembly since it is only necessary to place the knob 142 on the upper flattened end 1416 of the shaft, with the spring 149 in place, and then insert the knob into the hole in bracket 1611 from the under side thereof. The spring 149 is then compressed by pushing upwardly on the shaft, and the pivot projection 145 on the bottom of the cam 121 is moved laterally until it is over the hole in the lower bracket, when the spring 1419 snaps the projection 145 into the hole and the assembly is complete without any riveting or threading of parts together.

The knob 142 may be long as shown inthe drawings so that it extends into an opening in the breaker cover 13 where the adjustment is to be made from outside the breaker housing. lf it is not desired to make the adjustment is preferable, the adjustment is to be made from outside the housing as shown in FIG. l, the scale is placed on the cover 13 around the knob 142.

In order to retain the knob 142 and cam 121 in any position to which they have been adjusted, the flange 144 is provided with a plurality of notches 151 around its periphery, 'and there is a detent 15.3 pressed into the bracket 141 to engage one of the notches in each adjusted position. The knob 142 is movable by rst pressing it inwardly to clear the detent 153 and it may then be rotated to the desired position. The spring 149 then returns the knob to its outer position with the detent engaging another of the notches.

The adjusting rod 143 together with the cam 121 may be turned lby inserting a screw driver into a suitable slot in the outer end of the knob 142. When rotated clockwise (FIG. 4), due to the action of the cam surface 13) on the nose 129 of the lever 119, the latter is moved in a clockwise direction about its pivot 123 causing the adjusting screw 135 to move the armature 99 ciockwise about its pivot toward the magnet yoke 95 and away from the auxiliary pole pieces 115. In this manner the armature is adjusted and the adjustment is indicated on the scale 147.

By way of example, assuming that the breaker is rated to carry a normal load indefinitely without actuating .the trip device, the bimetal 45 may be calibrated to trip the breaker with an inverse time delay in response to overload currents of, for instance, between 100% and 300% of normal rated current. The tripping electromagnet may be calibrated and adjusted to instantaneously trip the :breaker in response to overload currents of, for instance, between 300% and 1100% of the rated current according to the adjustment of the armature 99. With 'the trip device calibrated to the above arbitrarily selected values, and with the cam 121 adjusted so that its highest step engages the nose 129 of the lever 119, the armature will be correspondingly moved closer to the magnet yoke 95, and the magnet will function to trip the breaker in response to overload currents of 300% or more of rated current. If the cam 121 is rotated to bring the lowest stepped surface into engagement with lthe lever 119, the armature will be moved away from the yoke 95, and the armature will not be attracted until the occurrence of an overload current of 1l00% of rated current.

The adjusting means just described is not only low in cost, but is very reliable in its operation. Gne reason for this is that the bottom of the cam rests on the rigid bracket 139 and it is the thickness of the material of the cam between its bottom and the steps 130 which determines the position of adjusting lever V119. Neither inward nor outward movement of the adjusting knob 142 can cause a change in calibration, or tripping of the breaker. `Inward movement of the knob is not transmitted to the cam because of the sliding or lost-motion connection between the knob and the upper end 146 of the shaft, but even if the knob were depressed so far as to make the spring 149 go solid, the cam could still not be moved because it is supported against the rigid stop 139. An outward pull on the knob is likewise ineffective to move the cam because of the lost motion connection and the rigid support of the upper bracket 141 on the flange 144.

Another advantage of the invention is that the adjusting knob may be reached from the front of the cover of the breaker and while the breaker cover is in place. Thus it is possible to make the adjustment when the breaker is installed in a switchboard,'control center or other enclosure, where the type of enclosure or the presence of a manual motor operating mechanism may make removal of the breaker cover difficult.

An additional advantage lies in the fact that the adjusting knob is not carried on a screw threaded member, so that it does not move in and out as it is turned. This keeps the top of the knob in the same relation to the top of the cover over it in each of its adjusted positions, so -that it is not recessed in the hole in the cover in one adjusted position and does not project a greater distance beyond the cover in another adjusted position. At the same time the construction is low in cost and easy to assemble because the knob may be moved axially of the shaft during assembly. The knob may be easily adjusted through the use of a tool, such as a screw driver, from the front of the breaker cover, and it remains in adjusted position because the spring returns it to its outermost position where the detent engages one of the notches.

While the invention has been disclosed in specific cmbodiment in accordance with the provisions of lthe patent statutes, it is to be understood that various changes in the structural details and arrangement of the parts thereof may be made without departing from the spirit of the invention.

We claim as our invention:

l. A circuit breaker having a first terminal at one end thereof and a second terminal at the other end thereof, said breaker comprising an energizing winding, a U- shaped magnet yoke disposed to be energized lby said winding, a movable armature having one end pivoted on one leg of said magnet yoke and the other end movable toward and away from the other leg of said magnet yoke, said armature moving generally along a vplane normal to a straight line between said rst and second terminals, adjusting means to vary the position of said armature relative to said magnet yoke and comprising a rotatable shaft, a cam mounted on one end of said shaft for rotation with said shaft, an adjusting knob mounted on the other end of said shaft for rotation with said shaft, support means fixedly connected to said magnet yoke and supporting said shaft for rotation, means preventing axial movement of said cam, a lever having one end engaging said cam, means on the other end of said lever engaging said armature, and rotation of said adjusting means causing said lever to vary the position of said armature relative to said magnet yoke.

2. A circuit breaker comprising a pair of terminals one disposed at each of two opposite ends of said breaker, an energizing winding, a U-shaped magnet yoke disposed to be energized by said winding7 a movable armature having one end pivoted on one leg of said magnet yoke and the other end movable toward and away from the other leg of said magnet yoke, said armature moving generally in a plane perpendicular to a straight line between said terminals, adjusting means to vary the position of said armature relative to said magnet yoke comprising a rotatable shaft, a cam rigidly mounted on one end of said shaft and an adjusting knob mounted on the other end of said shaft for rotation with said shaft, support means fixedly connected to said magnet yoke and supporting both ends of said shaft for rotation but preventing axial movement of said cam, a lever pivotally mounted on said support means and having one end. engaging said cam, adjusting means on the other end of said lever engaging said armature, manual rotation of said adjusting means moving said lever to vary the position of said armature relative to said magnet yoke, and said lever moving generally in a plane normal to said straight line.

3. A circuit breaker comprising an energizing winding, a U-shaped magnet yoke disposed to Ibe energized by said winding, an armature having one end pivotally supported on one leg of said magnet yoke and the other end movable toward and away from the other leg of said magnet yoke, adjusting means to vary the position of Said armature relative to said magnet yoke and comprising a rotatable shaft, a cam rigidly mounted on one end of said shaft and an adjusting knob mounted on the other end of said shaft for rotation with said shaft, support means comprising a pair of brackets fixedly connected to said magnet yoke and supporting both ends of said shaft for rotation but preventing axial movement of said cam, a lever pivotally mounted on one of said brackets and having one end engaging said cam, adjusting means on the other end of said lever engaging said armature, and manual rotation of said adjusting means causing said lever to vary the position of said armature relative to said magnet yoke.

4. A circuit breaker having a iirst terminal at one end thereof and a second terminal at the other end opposite said one end, said breaker comprising separable contact means and means releasable to effect separation of said contact means, a trip member operable to effect release of said releasable means, a trip device comprisingan energizing winding, a magnet yoke energized by said winding, an armature movable toward and away from said magnet yoke, said armature moving generally in a plane normal to a straight line between said first and second terminals, a trip rod actuated by said armature to engage and operate said trip member, adjusting means to vary the position of said armature relative to said magne-t yoke comprisingT a rotatable shaft and a cam mounted near the inner end of said shaft to rotate with said shaft, support means fixedly connected to said magnet yoke and supporting said adjusting means for rotation While preventing longitudinal movement of said shaft and said cam, a pivoted lever havin-g one end engaging said cam to be actuated by rotation of said cam, means on the other end of said lever engaging said armature, manually operable means on the other end of said shaft for rotating said cam to move said lever to vary the position of Vsaid armature relative to said magnet yoke, and said lever moving in a plane generally normal to said straight line.

5. A circuit breaker comprising separable contact means and means releasable to effect separation of said contact means, a trip member operable to effect release of said releasable means, a trip devicecomprising an energizing winding, a U-shaped magnet yoke energized by said winding, an armature having one end pivoted on one leg of said magnet yoke and the other end movable toward and away from the other leg of said magnet yoke, a trip rod actuated by said armature to engage and operate said trip member, adjusting means operable to vary the position of Said armature relative to said magnet yoke, said adjusting means comprising a rotatable shaft and a cam member rigidly mounted on one end of said shaft, support means comprising a pair of brackets tixedly connected to said magnet yoke and supporting said adjusting means for rotation While preventing longitudinal movement of said shaft and said Vcam member, a pivoted lever pivotally mountedon one of said brackets and having one end engaging said cam member to be actuated by rotation of said cam member, an adjusting screw on the other end of said lever engaging said armature, and manually operable means on the other end of sai-d shaft for rotating said cam' member to cause said lever to vary the position of said armature reiative to said other leg of saidmagnet yoke. n

6. A circuit breaker comprising separable contact means and means releasable to effect separation of said contact means, a trip member operable to effect release of said releasable means, a trip device comprising an energizing winding, a magnet energized by said winding, an armature movable to operate said trip device, adjusting means to vary the relative positions of said magnet and armature to change the magnet trip setting of the breaker, saidvadjusting means including a member rotatable about an axis to make said change in trip setting, a rotatable knob portion connected to said rotatable member for rotation therewith, said rotatable knob portion being mounted for at least-a-small movement axially while said rotatable member remains stationary, and indexing means releasable by axial movement of said knob portion.

7.A circuit breaker comprising separable contact means and means releasable to effect separation of said contact means, a trip member operable to eifect release of said releasable means, a trip device comprising an enerj gizing Winding, a magnet energized by said winding, an armature movable to operate said trip device, adjusting means to vary the relative positions of said magnet and armature and change the 'magnetic trip setting of the breakensaid adjusting means including a vmember rotatable about an axis and having a cam surface having por- -tions at varying positions in the direction along the axis of rotation of the member, a follower engaging said cam surface and making said changes in trip setting,a rotatabie knob portion connected to rotate said cam surface about its axis and having lost motion relative tosaid cam surface permitting axial movement of the knob portion without axial movement of the cam surface, and indexing means releasable by said axial movement of the knob portion.

8. In a circuit breaker, a trip device having an energizing winding traversed -by the current through thecircuit breaker, a magnet energized by said winding, an armature movableto operate said trip device, adjusting means including a shaft rotatable about its axis to vary the magnetic trip setting of the device, a-knob mounted for rotation with said shaft and for axial movement relative to said shaft, a rigid stop preventing axialmovement of said shaft upon axial movement of-said knob, and indexing means normally restaining said knob and shaft against rotation until said knob is moved axially of the shaft.

9. In a circuit breaker, a trip device having an energizing winding traversed by the ycurrent through the circuit breaker, a magnet energized by said Winding, an armature movable to operate saidtrip device, adjusting means including a shaft rotatable about its axis to vary the magnetic trip setting of the device, a knob mounted for rotation with said shaft and for axial movement relative to said shaft, a rigid stop preventing axial movement of vsaid shaft upon axial movement of said knob in one direction, a rigid stop preventing axial movement of the knob in the opposite direction, a spring biasing said knob and shaft in opposite directions against their re` spective stops, and indexing means permitting rotation of the knob and shaft upon movement of the knob axially relative to the shaft against the bias of said spring with the shaft held against axial movement by its rigid stop.

References Cited in the file of this patent UNITED STATES PATENTS 529,213 Sweet Nov. 13, 1894 2,491,959 Dyer Dec. V20, '1949 2,574,093 Edmunds Not/.6, 1951 2,586,326 Grissinger Feb. 19, 1952 V2,704,466 `Way Mar. 22, 1955 2,920,161 Dessert Ian. 16, 1959 

1. A CIRCUIT BREAKER HAVING A FIRST TERMINAL AT ONE END THEREOF AND A SECOND TERMINAL AT THE OTHER END THEREOF, SAID BREAKER COMPRISING AN ENERGIZING WINDING, A USHAPED MAGNET YOKE DISPOSED TO BE ENERGIZED BY SAID WINDING, A MOVABLE ARMATURE HAVING ONE END PIVOTED ON ONE LEG OF SAID MAGNET YOKE AND THE OTHER END MOVABLE TOWARD AND AWAY FROM THE OTHER LEG OF SAID MAGNET YOKE, SAID ARMATURE MOVING GENERALLY ALONG A PLANE NORMAL TO A STRAIGHT LINE BETWEEN SAID FIRST AND SECOND TERMINALS, ADJUSTING MEANS TO VARY THE POSITION OF SAID ARMATURE RELATIVE TO SAID MAGNET YOKE AND COMPRISING A ROTATABLE SHAFT, A CAM MOUNTED ON ONE END OF SAID SHAFT FOR ROTATION WITH SAID SHAFT, AN ADJUSTING KNOB MOUNTED ON THE OTHER END OF SAID SHAFT FOR ROTATION WITH SAID SHAFT, SUPPORT MEANS FIXEDLY CONNECTED TO SAID MAGNET YOKE AND SUPPORTING SAID SHAFT FOR ROTATION, MEANS PREVENTING AXIAL MOVEMENT OF SAID CAM, A LEVER HAVING ONE END ENGAGING SAID CAM, MEANS ON THE OTHER END OF SAID LEVER ENGAGING SAID ARMATURE, AND ROTATION OF SAID ADJUSTING MEANS CAUSING SAID LEVER TO VARY THE POSITION OF SAID ARMATURE RELATIVE TO SAID MAGNET YOKE. 